1. Field of the Invention
The present invention relates, in general, to microwave amplitude commutation feed devices, but more specifically it relates to a coaxial waveguide amplitude commutation network configured to operate compatibly with a scanning circular array antenna.
2. Description of the Prior Art
RF feed network implementation has been recognized as a critical problem area in scanning circular array antenna systems. A fundamental requirement of the rf feed network is the capability to commutate a desirable low-sidelobe amplitude distribution about the periphery of the circular array antenna in such a manner that a given number of radiating elements in a 180.degree. or smaller sector is excited at any instant of time. The rf feed implementation trade-off is usually one of physical complexity with the accompanying insertion-loss and tolerance control problems versus the ultimate desire of complete commutation capability of the ideal complex amplitude distribution, i.e., the desirable amplitude distribution magnitude and the requisite phase characteristics for plane-wave generation from a circular sector boundary. The network implementation problem is one of practical rather than theoretical realizability and a number of rf feed networks have been developed to cope with the circular array antenna geometry. Among these have been the R-2R lens feed [1], the Butler Matrix feed [2], the transfer switch matrix feed [3], and the waveguide commutator feed [4]. Various other feasible circular array feed configurations have also appeared in the prior art but they can essentially be categorized as one or a combination of the basic four feed types aforementioned.
The referenced feed networks all possess the desired capability of performing the amplitude commutation function, but each configuration is associated with one or more undesirable physical or performance attributes which most frequently reside in the areas of design complexity, rf output amplitude and phase tolerance control, insertion loss, or bandwidth limitations. The coaxial waveguide commutation feed network, according to the present invention, is characterized by simplicity of configuration with a resultant significant overall improvement in the critical cited performance areas. A key element of the coaxial waveguide commutation feed network is a coaxial waveguide commutator similar in performance capability to the radial waveguide design developed previously [4] but without the bandwidth limitations inherent in this nonuniform transmission-line device [5]. Some unique features of the coaxial waveguide commutation feed network are a simple coaxial input probe excitation implementation technique and the capability of an rf bandwidth in excess of 30-percent with respect to the mid-band frequency at which the coaxial waveguide commutator is tuned. The tuning devices of the present invention can be employed to tune to different mid-band frequencies while maintaining the 30-percent rf bandwidth operation. A dominant TEM mode and a pair of orthogonal TE.sub.11 modes excited within the coaxial waveguide commutator are employed to generate a commutatable low-side-lobe amplitude distribution about the periphery at its output ports.
An exemplary sample of a network for supplying rf power to a plurality of radiators in a phased array according to a desired distribution pattern is disclosed in U.S. Pat. No. 4,005,379 to Lerner entitled "R.F. Distribution Network for Phased Antenna Array," issued on Jan. 25, 1977 from application Ser. No. 628,469, filed Nov. 4, 1975. In Lerner, a TEM mode and a pair of selectively phase shifted TE.sub.11 modes are derived and applied to input ports of a cavity resonator to produce the desired rf power distribution at a plurality of output ports. The cavity resonator is a cylindrical member in which the output ports are arranged circumferentially about the periphery and axially spaced from the TE.sub.11 mode input ports.
Lerner uses a single probe to excite the dominant TEM mode in the coaxial section of the cavity resonator, and a pair of probes to excite a single TE.sub.11 mode, plus another pair of probes to excite an orthogonal TE.sub.11 mode. Using only a single probe pair to excite a TE.sub.11 mode limits the bandwidth capability compared to four-probe excitation in a coaxial waveguide. Consequently, there is a need in the prior art to configure a coaxial waveguide commutation network to including four-port feeding to inhibit the higher order TE modes thereby increasing the bandwidth capability.
Lerner also uses a 3db coupler, a pair of phase shifters, and a pair of baluns to achieve the excitation of the pair of TE.sub.11 modes. This arrangement appears to be inherently more difficult to align and to control so as to produce the selective phasing required between the pair of TE.sub.11 modes and the TEM mode so that the low-sidelobe amplitude distribution can be rotated about the output ports. Hence, there is a need in the prior art to configure networks of the kind discussed to have broadband capability while maintaining simplicity in configuration and alignment, and reliability in performance.
Additionally, Lerner does not incorporate any special impedance matching techniques to provide a smooth impedance transition for the dominant TEM mode from the input ports to the output ports of the commutator. Thus, there is a need in the prior art to provide a smooth impedance transition to further enhance bandwidth.
The prior art, as indicated hereinabove, include some progress in implementing feed networks to cope with the special problems inherent in the circular array antenna geometry. However, insofar as can be determined, no prior art network or method incorporates all of the features and advantages of the present invention.